The present invention relates to a magneto-resistive device, and a magnetic head, a head suspension assembly and a magnetic disk apparatus using the same.
With the trend to a larger capacity and a smaller size of hard disk drives (HDD), heads are required to have a higher sensitivity and a larger output. To meet these requirements, strenuous efforts have been made to improve the characteristics of GMR heads (Giant Magneto-Resistive Head) currently available on the market. On the other hand, intense development is under way for a tunnel magneto-resistive head (TMR head) which can be expected to have a resistance changing ratio twice or more higher than the GMR head.
Generally, the GMR head differs from the TMR head in the head structure due to a difference in a direction in which a sense current is fed. A head structure adapted to feed a sense current in parallel with a film surface, as in a general GMR head, is referred to as a CIP (Current In Plane) structure, while a head structure adapted to feed a sense current perpendicularly to a film surface, as in the TMR head, is referred to as a CPP (Current Perpendicular to Plane) structure. Since the CPP structure can use a magnetic shield itself as an electrode, it is essentially free from short-circuiting between the magnetic shield and a device (defective insulation) which is a serious problem in reducing a lead gap in the CIP structure. For this reason, the CPP structure is significantly advantageous in providing a higher recording density.
Other than the TMR head, also known as a head in CPP structure is, for example, a CPP-GMR head which has the CPP structure, though a spin valve film (including a specular type and dual spin valve type magnetic multilayer films) is used for a magneto-resistive device.
Any type of CPP-based heads has an upper electrode and a lower electrode for supplying a current to a magneto-resistive layer formed on a base, formed on the top (opposite to the base) and on the bottom (close to the base) of the magneto-resistive layer, respectively. Generally, for reasons of manufacturing process, the base formed with the magneto-resistive layer is left in the atmosphere after the magneto-resistive layer is formed and before the upper electrode is formed. In this event, for preventing the top surface of the magneto-resistive layer from being oxidized in the air to damage the characteristics of the magneto-resistive layer such as an MR ratio, a non-magnetic metal layer, referred to as a “cap layer”, is previously formed as a protection film on the top surface of the magneto-resistive layer. The non-magnetic metal layer is preferably made of a material insusceptible to oxidation or a material having a low resistance even if it is oxidized. Ru, Rh, Pt, Au, Ta or the like is used for the non-magnetic metal layer. Then, in the CPP-based head, the upper electrode is electrically connected to the magneto-resistive layer through the non-magnetic metal layer.
A head utilizing a spin valve film or a TMR film is applied with a biasing magnetic field to a free layer in a track width direction, whether it is in CIP structure or in CPP structure, in order to suppress Barkhausen noise. Generally, in the CIP structure, a resist mask used for milling for defining the track width is utilized as it is to form hard magnetic films made of CoCrPt or the like adjacent to both sides of a magneto-resistive layer as magnetic domain control films. This is referred to as an “abutted structure”. Like a CIP-GMR head, the CPP structure also employs the abutted structure to apply a biasing magnetic field to a free layer (see, for example, JP-A-2000-30223 corresponding to U.S. Pat. No. 6,344,955, JP-A-2001-14616 corresponding to U.S. Pat. No. 6,545,848, and the like). In this way, in any type of head, the biasing magnetic field is generally applied to the free layer through the abutted structure.
On the other hand, an article by Nakashio et al., entitled “Longitudinal bias method using a long distance exchange coupling field in tunnel magnetoresistance junctions”, Journal of Applied Physics, Vol. 89, No. 11 (Jun. 1, 2001), pp 1–3 and JP-A-2001-68759 have proposed magneto-resistive elements (TMR elements) which have an antiferromagnetic layer made of IrMn or the like laminated on a free layer of a magneto-resistive layer as a magnetic domain control film for applying a biasing magnetic field to the free layer. In this TMR element, a non-magnetic metal layer made of Cu or the like is formed on the free layer on a tunnel barrier layer, and the antiferromagnetic layer is formed on the non-magnetic metal layer. According to this TMR element, an exchange bias magnetic field of the antiferromagnetic layer is applied to the free layer in the track width direction through the non-magnetic metal layer as a biasing magnetic field. As a result, the magnetic domain of the free layer is controlled to suppress the Barkhausen noise without fixing the magnetization direction of the free layer.
Conventionally, in a magnetic head which employs the abutted structure, an insulating layer made of Al2O3 or SiO2 is disposed not only near an end face of a magneto-resistive layer but also over a region quite far away from the end face on the side of the magneto-resistive layer opposite to a magnetic recording medium side (ABS side), on which the hard magnetic film constituting the magnetic domain control layer for applying a biasing magnetic field to a free layer is not disposed. In a magnetic head which employs the structure disclosed in the above cited article and JP-A-2001-68759, since the antiferromagnetic layer is laminated on the magneto-resistive layer as a magnetic domain control layer for applying a biasing magnetic field to the free layer, an insulating layer made of Al2O3 or SiO2 is formed not only near an end face of the magneto-resistive layer but also over a region far away from the end face over the entire periphery other than the side of the magneto-resistive layer closer to the magnetic recording medium side (ABS side).
In the CPP-based head such as the TMR head, the magneto-resistive layer is supplied with a current through the upper electrode and non-magnetic metal layer (cap layer), so that a good electrical contact must be maintained between the upper electrode and non-magnetic metal layer to reduce the resistance. However, when the base formed with the magneto-resistive layer and non-magnetic metal layer is left in the atmosphere, the surface of the non-magnetic metal layer is oxidized in the air. Even if a material insusceptible to oxidization is used for the non-magnetic metal layer, a slight oxide film or surface adsorption layer is inevitable. Thus, if another layer such as an upper electrode is formed on the oxidized non-magnetic metal layer, a good electrical contact cannot be maintained between the upper electrode and non-magnetic metal layer. To solve this inconvenience, the surface oxide film is removed from the non-magnetic metal layer by dry etching (such as sputter etching, ion beam etching or the like) within the same vacuum chamber in which the upper electrode and the like are deposited, prior to the formation of another layer such as the upper electrode on the non-magnetic metal layer.
However, when the surface oxide film is fully dry etched for a lower resistance during a removing step, the magneto-resistive layer is seriously damaged by an ion beam. For example, with the TMR head, an extremely thin tunnel barrier layer (for example, 1 mm or less in thickness) is seriously damaged by the ion beam to cause an extreme reduction in MR ratio and occasional failure in a utilization as a magnetic head.